1. Field of the Invention
The present invention relates to a method for detecting the concentration of an impurity in a crystal grown out from a molten liquid in a crucible and a production method of a single crystal, particularly the detecting method of impurity concentration in a single semiconductor crystal such as silicon, the production method and the pulling up apparatus for the single silicon crystal.
2. Description of the Related Art
A single crystal of silicon used in an instrument using a semiconductor is, in general, made through the pull method (CZ method), the floating zone melting method (FZ method) or the method of pulling up the crystal under a magnetic field (MCZ method). The CZ method and MCZ method are to melt a high purity polycrystalline silicon in a quartz crucible and pull up a growing single silicon crystal from a molten silicon liquid. Therefore, oxygen enters the molten silicon liquid through dissolving SiO.sub.2 from the quartz crucible. The oxygen thus entered is transferred to just below the crystal, diffusing along the flow of the molten liquid or diffusing through in the molten liquid and caught in the single crystal penetrating through a growing interface of the crystal.
An ingot of the single silicon crystal obtained by the CZ method and the like is sliced into thin wafers. The oxygen caught in the crystal changes the electrical resistance of the wafer, so the wafer having a required electrical resistance can not be obtained. The oxygen in the crystal may cause degradation of a wafer due to precipitation of oxides during high temperature processing of the silicon wafer. On the contrary, the oxygen caught in the crystal increases the crystal strength, blocking dislocation movement to increase so-called adherence strength, and also has a merit to reduce the deformation of the wafer (warp) resulting from heat processing. The oxygen in the crystal is also applied to intrinsic gettering for trapping the impurity in the crystal to the outside of the elemental area (device forming area).
A surface layer of the silicon wafer is used as a device forming area to form a device and the existence of impurity gives a device unfavorable effects. Therefore it is necessary to eliminate the impurity in the device forming area by some means. So an oxidized precipitate caused by oxygen in the crystal is precipitated below the device forming area through heat processing of the silicon wafer and the impurities such as heavy metals are caught by the precipitates so that the impurities can not move into the device forming area by means of so called intrinsic gettering and the pure device forming area can be obtained.
The intrinsic gettering by oxidized precipitates requires a precise control of oxygen content in the crystal and a uniform distribution of the oxygen. Moreover, the heat processing to separate the oxidized precipitates differs according to each semiconductor maker and the contents and sizes of the oxidized precipitates required by these makers are different with each other. So the wafer maker is required to meet the requirement from customers on the oxygen content in the crystal. Accordingly, it is very important for the production of a single silicon crystal to check the oxygen concentration in the molten liquid which pulls up the single crystal and to control the amount of oxygen taken into the crystal.
Most oxygen dissolved in the molten silicon liquid from a quartz crucible is known to evaporate as silicon oxides from the free surface of the molten liquid into the furnace atmosphere (in most cases in argon (Ar)). Then, oxygen concentration in the single silicon crystal is determined by the way of distribution of the oxygen whose amount is the subtraction of evaporation amount from dissolved amount from the quartz crucible, in the molten liquid inside the quartz crucible. Particularly it is determined by the oxygen distribution pattern in the growing interface of the crystal and the concentration level. And the oxygen distribution pattern in the growing interface of the crystal greatly affects the distribution of the oxygen concentration in the surface orthogonal to the axis of the single crystal ingot, and the concentration level greatly affects the height of oxygen concentration.
But it is impossible, at present, to directly control the amount of oxygen caught in the crystal while pulling up the crystal. Therefore, oxygen concentration control in a single silicon crystal by current CZ method is controlled by adjusting the oxygen distribution pattern just below the growing interface and the concentration level with changing the transportation style of oxygen by flow of the molten liquid using the following items as a parameter. The items used as a parameter are a heating value of the heater, number of revolution of the crucible, number of revolution of the crystal and furthermore, so called H/Z structure such as shape and size of the heater or the heat insulator and position of the heater, and in the MCZ method, these items and pattern and strength of the magnet field being applied. And it is reported that in the free surface of the molten liquid, the oxygen concentration becomes lower due to evaporation of the silicon oxides, the molten liquid in the low oxygen content free surface lowers the oxygen concentration at ends of the crystal, crawling underneath the crystal growing interface and makes the distribution of oxygen concentration in the wafer surface uneven.
But the method to control the oxygen concentration in the crystal by changing the flow of the molten liquid has been empirically carried out to a great extent from experience due to the difficulty of detecting the flow of the molten liquid and not to directly control the amount of oxygen caught in the crystal. Besides, the conventional oxygen control method is not clear on the extent of influence of the molten liquid flow upon the oxygen concentration in the crystal so the precise concentration control has been difficult.
On the contrary, dopant is added into molten silicon liquid to control electrical resistant rate of the single silicon crystal, this dopant also evaporates from the free surface of the molten liquid as oxides or silicide, the distribution pattern and the level of concentration of the dopant directly below the growing interface change in accordance with the flow of the molten liquid or diffusion. Then, same as in the case of oxygen above described, the distribution pattern and concentration level of the dopant directly below the growing interface is adjusted, using the flow of the molten liquid as a parameter, electric resistance of the crystal is controlled. But as described before, the method of controlling the flow of the molten liquid is difficult for making the electric resistance of the crystal as predetermined because the dopant concentration in the crystal can not be controlled precisely. As a result, the resistance distribution in the surface orthogonal to the axis of the crystal which is pulled up or the resistance level along the axis of the crystal are greatly changed and a wafer having a uniform quality can not be obtained.